processes with identical efficiencies. Calibration of instruments using suitable standards is thus the
normal approach to practical measurements.
Bombardment Reactions and the Growth of Radioactivity
The number of naturally occurring radionuclides is limited and few are of analytical value. For the
majority of purposes artificial radionuclides are manufactured. Bombardment reactions are generally
used in their production. A suitable target material is exposed to an intense flux of the appropriate
particles in a nuclear reactor or particle accelerator such as a cyclotron. Thermal neutrons in the reactor
are efficient in producing (n, γ) neutron capture reactions e.g.. The products of these
reactions will have an excess of neutrons and generally decay by (β–, γ) emission. The major
disadvantage is that the radioactive atoms will always be diluted with many non-radioactive atoms and
chemical separation is not possible. (n, γ) reactions are, however, usefully exploited in neutron
activation analysis (p. 472). With fast neutrons, proton, deuteron or alpha particle bombardment a
change in atomic number accompanies the reaction and chemical separation of the 'carrier free'
radiotracer becomes possible, e.g.
The growth and decay of a nuclide in the target during a bombardment reaction may be exemplified by
a generalized sequence
The growth of Y will proceed at a rate dependent upon the intensity f of the particle flux, the amount NX
of the target present and the nuclear cross section σX for the reaction (Figure 10.5). After time t, the
activity induced due to Y (i.e. NYλY) will be given by
where σX is measured in barns (1 b = 10–^28 m^2 ). If t > 5(t1/2)Y the exponential term may be ignored and
equation (10.13) becomes
i.e. the saturation activity (Figure 10.5). Growth and decay considerations are important in activation
analysis methods. It should be noted that nuclear reactions are specific for a particular nuclide. Thus NX
represents the number of atoms of the nuclide, which differs from the total number of atoms of the
element when the element is not monoisotopic.